The members of the Circoviridae are the smallest autonomously replicating DNA viruses known and are pathogens in plants, avians and mammals. The form of virion DNA is unique as it is a single stranded circularized molecule of roughly 1800 bases. While the pathogenicity of the plant and avian circoviruses is recognized, only recently has the pathogenicity of circoviruses in swine been realized; the disease potential of recently described human circoviruses remain to be determined. The long term goal of our multi-institutional and international research group is to elucidate pathogenic mechanisms of disease caused by infection with porcine circovirus type 2 (PCV-2), which can result in progression to fatal postweaning multisystemic wasting syndrome (PMWS) in swine. It is our central hypothesis that both pathogenic mechanisms and viral virulence determinants of PCV-2 can be elucidated by confirming that unencapsidated viral single-stranded (ss) circularized DNAs circulate in high copy numbers in the plasma of virus-infected gnotobiotic swine and that these unencapsidated viral DNAs are directly infectious and pathogenic for gnotobiotic swine. The biological significance of this will be assessed by local wound and oronasal inoculations into swine temporarily protected from infection with encapsidated virions by maternal-origin antibodies. Secondly, we will test the virulence potential of archival (ar) PCV-2, as a reconstructed viral isolate or as chimeric virus constructs. This virus has been reconstructed from cloned viral DNA recovered from porcine tissues collected many years prior to the appearance of PMWS in swine. ArPCV-2 has been sequenced and a consistent difference in nucleotide sequence which translates into a linear 3 amino acid sequence difference in the nucleocapsid protein between ar- and contemporary pathogenic (pa) PCV-2. It is our hypothesis that paPCV-2 acquired this change by mutation and this event may explain the emergence of new pathogenic PCV-2 in recent years. Collectively, these data will provide critical insight into pathogenic mechanisms of disease mediated by mammalian circoviruses and will provide key guidance for the future study of human circovirus pathogens.